Dry eye syndrome (DES), also known as keratoconjunctivitis sicca (KCS), is a condition that occurs due to loss of water from the tear film and is one of the most common complaints seen by optometrists. Studies have found that DES is common in about 15% of patients over the age of 50, with prevalence increasing with age. Dry eye in general is caused by any condition that increases tear film evaporation or by any condition that decreases tear production. For example, evaporation may be increased as a result of having larger eyes (i.e., having more surface area for evaporation to occur from). Also, tear production may decrease from any condition that decreases corneal sensation, such as long term contact lens wear, laser eye surgery, trauma to the 5th nerve, and certain viral infections, etc.
The treatment of DES depends on the severity of the condition. Some patients find relief through the use of various artificial tears. Others utilize supplements containing Omega-3. Still others resort to the insertion of punctual plugs to stop the drainage of tears. Effective treatment, however, begins with effective diagnosis.
In order to diagnose DES, it is useful to determine the osmolarity of the tears in the affected eye. Osmolarity is the measure of the concentration of osmotically active species in a solution, and may be quantitatively expressed in osmoles of solute per liter of solution. It is known that when the tear film loses water, salt and protein concentrations increase relative to the amount of water, resulting in increased osmolarity. Therefore, in order to diagnose and treat DES patients, it is desirable for a treating physician to quantify the osmolarity of a sample tear fluid.
Current techniques for measuring osmolarity involve osmotic pressure measurement, freezing point depression analysis, vapor pressure measurement, and electrical resistance measurement. In one approach, an osmometer is used to measure the osmotic pressure exerted by a solution across a semi-permeable membrane. The osmotic pressure can be correlated to the osmolarity of the solution.
In another approach, the osmolarity of a sample fluid may be determined by an ex vivo technique that involves analyzing the freezing point of the sample fluid. Deviation of the sample fluid freezing point from 0° Celsius is proportional to the solute level in the sample fluid, and is indicative of the osmolarity.
In a further known ex vivo technique, a piece of filter paper is placed under the patient's eyelid to absorb tear fluid. The paper is removed and placed in an apparatus that measures a dew point. The reduction in dew point proportional to that of water can be converted to an osmolarity value.
Lastly, osmolarity may be determined by measuring the conductivity of a fluid sample. The measurement may be made in vivo by placing electrodes under the eyelid. Alternatively, the measurement may be made ex vivo by collecting a sample from the patient and transferring it to a measurement apparatus.
Known techniques for measuring osmolarity, such as those described above, rarely produce accurate or consistent results because they suffer from problems including, for example, inducement of reflex tearing and evaporation of fluid samples. Reflex tearing occurs when the tear glands of the patient are stimulated during tear collection. The stimulation produces extra amounts of liquid, which can lead to false readings (e.g., too high water content). Conversely, when very small samples are taken to avoid reflex tearing, the small samples often immediately begin to evaporate, which can lead to false readings (e.g., too low water content).
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.